1. Field of the Invention
The present invention relates to an electronic still camera and particularly to the picture-taking operation of the camera.
2. Related Background Art
A typical construction of the circuit of an electronic still camera is shown in FIG. 5.
In the arrangement shown in FIG. 5, an object image is focused on a light receiving surface of a state image pickup element 104 through a lens 101 under an exposure condition selected by a diaphragm 102 and a mechanical shutter 103.
An output produced from the state image pickup element is converted into a video signal based on a format by a signal processing circuit 105 and then FM-modulated by a recording circuit 106. The modulated signal is recorded in a magnetic recording disk at a recording portion 107. A timing pulse generator 108 generates a driving pulse for the image pickup element 104 and a timing pulse required for the signal processing circuit 105. An operation control unit (central processing unit, CPU) 109 controls all of the timings for a series of picture-taking operations.
In the above-shown conventional apparatus, a picture is taken through the following operational steps whose time chart is shown in FIG. 6.
A picture-taking operation starts with falling of a release signal and a power supply for every circuit shown above is switched ON.
At the falling of a 1 CMg signal, the leading shutter blade of the mechanical shutter starts running and at the falling of a 2 CMg signal, the trailing shutter blade starts running. In other words, an exposure is started at the start of running of the leading shutter blade and is ended at the completion of running of the trailing shutter blade.
In FIG. 6, Vp signal is a signal synchronized with a vertical synchronizing signal with a delay of about 700 .mu.sec. A TG pulse signal later described is within the Vp pulse. The start of running of the leading shutter blade mentioned above is in synchronism with a time immediately after a Vp signal.
PG signal is a signal for the start position of the magnetic disk of the recording portion. The signal is so controlled that revolution of the disk is coincident with one period of the vertical synchronization time.
RESET signal is a signal used for the phase shift of a PG signal and a Vp signal during recording and for another similar purpose. At the falling of this RESET signal, a synchronizing signal from the odd field side of the vertical synchronizing signal as well as every timing pulse start.
Odd TG and even TG signals are signals for readout of charge from the solid state image pickup element. The odd TG signal is used for readout of odd field pixels and even TG signal for readout of even field pixels. CCD control signal is a signal for masking the charge readout signal. The latter signal is masked for the time from exposure start to a time immediately before recording start. The time during which the exposure charge is read out from the image pickup element and recorded in the magnetic disk is determined by a recording time signal.
In the prior art apparatus, as seen from the timing chart, a discharging of unnecessary charge on the solid state image pickup element (especially discharging from its light-receiving part) is carried out during the time of from release to exposure start for the purpose of removing any noise, as caused by dark current.
Before releasing, since the power supply to the image pickup element is not ON, the element is in a thermally balanced state. In other words, the wells of the light-receiving part and transfer part are full of charge. In order to completely discharge it so as to empty the wells after releasing, readout of charge must be carried out more than at least three times continuously after releasing. To this end, usually, the readout operation is carried out at the rate of once per about 33 ms. (two vertical synchronization periods) in the frame accumulation mode, with odd field pixels and even field pixels being read out alternately, each for one vertical synchronization period. The repetition rate of odd TG signals and of even TG signals is half that of the vertical synchronization signals. This means that the drainage of unnecessary charge on the image pickup element must be carried out for a long time, more than 100 ms from releasing.
The readout of charge of odd field pixels is effected in response to an odd TG pulse and the readout of charge of even field pixels is effected in response to an even TG pulse. However, this drainage of unnecessary charge in the operation mode of frame accumulation involves the following problem.
As previously noted, in order to fully discharge the unnecessary charge, at least three times of readout are required for each pixel field. Therefore, in this case, an exposure can be started only after each pixel field has been read out at least three times or more. This means that the time passed from releasing to exposure start (time lag) is long. In order to shorten the time lag, the number of times of readout must be decreased. But, in this case, the remaining unnecessary charge is increased and, therefore, the noise increases.